Diodes function ideally as “one-way valves” in electronic circuits, allowing current to pass unhindered in one direction and blocking current in the opposite direction. Solid-state diodes are generally constructed from semiconductor crystals with different layers of the crystal having different electrical properties. The most basic semiconductor diode is formed by adjoining two semiconductor crystals: one, called a P-type, that has been doped with atoms so that the majority of charge carriers are mobile positive holes, and the other, called an N-type, that has been doped with atoms so that the majority of charge carriers are negative electrons. This so-called “P-N junction” is connected to the rest of the circuit by attaching an anode material to the P-type semiconductor and a cathode material to the N-type semiconductor.
More complicated diodes are constructed by having one or more layers of N-type semiconductor and/or one or more layers of P-type semiconductor, with different layers having different amounts of mobile charge carriers. In the case of a Schottky diode, the P-type material is omitted entirely with the anode connected directly to the N-type semiconductor. The physical geometry of the diode can also be adjusted, from a simple stack of the various layers to much more complicated arrangements.
An ideal diode serves three distinct functions. First, allow current to flow from the anode side to the cathode side with no drop in potential, and hence no loss of electrical energy. When current is flowing in this direction the diode is said to be in the “forward biased state.” Second, completely block all current flowing in the opposite direction regardless of the applied voltage, which is called the “reverse biased voltage.” Third, instantaneously switch between the first two behaviors, that is, immediately transition between the forward “on” state of allowing current to flow, and the reverse “off” state of blocking all current. This transition is called “commutation,” “recovery,” or “reverse recovery.”
However, diodes do not function ideally. First, there is usually some small drop in potential when the diode is in the forward biased state. Second, the diode allows a nonzero leakage current to flow in the reverse biased state and this leakage current increases as the reverse biased voltage increases. Further, when the reverse bias voltage reaches a certain breakdown voltage the reverse current increases dramatically. Third, a real diode takes time to switch between the on state and the off state. When conducting, the diode is flooded with electrons and holes moving in opposite directions. In order for the diode to transition to the off state this excess “stored charge” needs to be removed from the diode during what is called the “reverse recovery time.”